Looking for feedback, help and opinions on an issue re-wiring my 1964 Mercury Monterey. Note: I have not wired a car before, but I'm not a stranger to cars and wiring seems mostly straightforward. I'm almost ready to test and start 'er up, after I finish the instrument cluster. I'm having some confusion installing an idiot light for the alternator. I have searched the HAMB, HotRodder's Bulletin Board, and the Galaxie forum at FordMuscle.com to understand how this should work...only found a couple threads that were helpful, so I'm starting a thread to get your views. thank you in advance for your assistance. Background 1. Installing a Rebel Wire 9+3 harness (really nice product!) 2. Replacing the externally-regulated Ford alternator with an internally-regulated GM 12SI unit. 3. Replacing the Merc's original ammeter with an idiot light -- the ammeter seemed to be the epicenter of a colossal harness meltdown, thus the re-wiring job. 4. I'd rather not install a voltmeter: I want to maintain the original look of the dash cluster if possible. (Pic 1) 5. Intend to use a 12V LED for the idiot light. Again, doing my best to maintain the original look of the dash cluster, this tiny LED is mounted by drilling through the face of a spare ammeter (Pic 2) obtained from HAMBer ArizonaMoparGold, thanks Bob! So I still have the original, unmolested ammeter if I need to go a different direction. 6. from what I have read, and from talking with Bob at Rebel Wire (great customer support!), the idiot light is installed by cutting into the excitor wire on its way to the alternator. To mimic the resistance of an 1895-type 12V idiot light, like Ford installed on the similar Galaxie, I wired a 10-ohm, 10 watt resistor in parallel with the LED light. (pic 3) my understanding is that I need about 10 ohms resistance so the warning light only comes on when an undercharge situation at the alternator/regulator occurs. The resistor is a ceramic type, mounted to a Radio Shack wafer board that I screwed to the back of my instrument cluster....the LED and "jumper wires" that will connect to the excitor wire are soldered to the resistor on the wafer board. (Note: hard to tell from the photo, but I used 1/2" spacers to keep the wafer board up off the cluster and provide room for air to circulate.) I tested the setup by connecting it to the battery and ground. the LED lights up, looks good, nothing caught on fire or exploded. Disconnected from power, touched the resistor with my finger and it's hot as hell. Questions: 1. Has anyone done this before? 2. is my approach of wiring a resistor in parallel to the LED all wrong? or did I simply fuck up by testing it directly to the battery? 3. is the LED just too lightweight for the current on the excitor wire? all LEDs have such tiny wires! 4. same question on the wafer board? is this thing just too lightweight? 5. I know the resistor will put off a little heat but DAYUM it got hot fast! 6. am I fucking up by not insulating the wafer board solder points somehow? pic 1 - Merc instrument cluster pic 2 - Merc ammeter with LED installed pic 3 - back of cluster with LED wired to resistor. you can still see the black smoke marks from the harness meltdown! Thanks everyone!
If I understand correctly (and I may not :> ) you've now routed all the current draw through a resistor, instead of through the original ammeter. I'm not sure how that is any better or safer than before. One thing you could do is get that ammeter retrofitted as a voltmeter, but keeping the original look. I sent my entire gauge cluster to redline gauge works, and got the oil, temp, ammeter, and fuel gauges retrofiited to 12 volts and the ammeter replaced with a voltmeter. Everything looks exactly the same but has new internals. It cost me 500$ canadian all totalled and shipped. If you just need the ammeter done it should be not nearly that much, and that was when 1$ us was like a million canadian.
I dunno. as I said, I don't know wtf I'm talking about...but I don't think the excitor circuit carries much current. found this about the excitor circuit. Might be where I got started down this path in the first place Alternator Light - http://www.inliners.org/ubbthreads/ubbthreads.php?ubb=showflat&Number=56339&PHPSESSID=9679892821efe0333c6e880fb1e844ca says (on a discussion of functions of the excitor circuit) "I assume you are talking about a 10SI or 12SI alternator. You looked at the wiring diagram for a '70s car and you did not and should not have seen a diode. You should have seen a resistor. In most cases it is a resistor wire, much like a ballast resistor wire but without the cloth insulation. Sometimes the wire is just listed as "Res. wire" with no symbol. The resistor serves two purposes. It allows more current to flow to the Alt so that it begins to charge without reving the engine. The second purpose of the resistor is in case the bulb should burn out, the exciter circuit continues to function. In the original wiring, one side of the resistor is connected to a Acc. connection on the ignition switch. On most retro-fits on older cars, you don't have an extra connection on the ignition switch. Go to Radio Shack and get a 10 ohm, 10 watt resistor and wire it in parallel across the Gen lamp. This will duplicate what GM did and your problem will be solved." I suppose I could substitute the appropriate resistor wire for the Radio Shack resistor?
I would not worry about the heat at the resistor. You left the current flowing longer than it ever will when it is installed and working with the alternator. When the alternator is charging there is no current flow. The resistor will have 12 volts on each side with no path to ground. That is why using an incandescent light bulb the light goes out after the car starts and the alternator is charging. There is no current flowing. It only flows to "excite" the alternator (this is like priming a pump).
The Jalopy Journal
